It is known for current steering digital to analogue converters (DACs) to be used to control biasing of highly critical radio frequency (RF) elements in wireless communication units and applications. Such current steering DACs are required to have ultra low noise characteristics. In particular, ultra low flicker noise levels are necessary when such a DAC is used in order to control output power of a power amplifier in, say, a radar system.
A conventional technique for achieving low frequency noise conditions for a current steering DAC is to use a passive RC (resistance-capacitance) filter on the reference voltage of a current mirror within the DAC. The higher the RC product of the RC filter, the lower the low frequency noise characteristics. However, a problem with such a technique is that the higher the RC product of the RC filter, the higher is the time constant thereof and, thus, the greater the length of time is required to recover a stable state when the DAC digital input code is changing. Thus, a high RC product within the RC filter limits the speed of the DAC.